The neutralization of sulfated alcohols in a single stage, as is carried out in the known art, requires extreme stirring and cooling conditions in order to prevent local phenomena (at the micellar level) in the form of temperature increase and/or preferential contact of the monoester of sulfuric acid with the dilution water and/or reaction with consequent degradation. This degradation can be represented by the following reactions: EQU R--O--SO.sub.3 H+H.sub.2 O.fwdarw.R--O--H+H.sub.2 SO.sub.4 ( 1) EQU 2.R--O--SO.sub.3 Na+H.sub.2 SO.sub.4 .fwdarw.ROSO.sub.3 H+Na.sub.2 SO.sub.4( 2)
wherein R indicates the alcohol radical. The first term of the left-hand side of reaction (1) is therefore a monoester of sulfuric acid. The first term of the right-hand side of reaction (1) is the initial alcohol obtained by degradation of the monoester of sulfuric acid in contact with the water present in the reaction mixture. As can be seen, the regression of the monoester of sulfuric acid to alcohol is accompanied by the formation of sulfuric acid. Reaction (1) is favoured by increased contact time between the monoester of sulfuric acid and the water present in the reaction mixture, and also by increased mixture temperature. In reaction (2) the first term of the left-hand side represents the monoester of sulfuric acid salt obtained by the neutralization reaction, which salt, in the presence of the sulfur trioxide obtained from reaction (1), becomes reconverted into the initial monoester of sulfuric acid, with the formation of sodium sulfate. This second reaction is also favoured by increased reagent contact time and increased temperature.
Even if the aforesaid reactions take place in only a small part of the total mass of mixture being neutralized, the product suffers the following negative consequences:
(i) increase in the percentage of sulfated compound in the product from the neutralization stage, to the detriment of the true degree of conversion obtained in the previous sulfation stage;
(ii) increase in the NaOH consumed;
(iii) increase in the quantity of sodium sulfate present in the final product, with a consequent considerable increase in its viscosity (any viscosity increase of the product worsens stirring and heat transfer conditions, thus leading to further degradation).
Furthermore, the content of inorganic salts is related to the minimum required value for the commercial product, and must in practice be no more than 1.5-2%, compatible with the content of sulfuric acid originally present in the monoester of sulfuric acid and which is produced during the sulfation stage.
To obviate these drawbacks, represented by the above reactions (1, 2), the known state of the art provides for the use of high power mixers and large circulation pumps for cooling the mixture subjected to the neutralization process. Neutralization is therefore carried out in a plant which comprises: a high power mixer; a pump of high capacity and head; and a heat exchanger of the plate or other high efficiency type.